The present invention relates to a new process for the preparation of riboflavin starting from D-glucose.
Processes for the preparation of riboflavin are known. The process usually is carried out on a large industrial scale and uses D-glucose as the starting material, which is oxidized to D-arabonic acid, the D-arabonic acid is epimerized to D-ribonic acid and the D-ribonic acid is then converted into D-ribonolactone, from which D-ribose is formed by amalgam reduction, the D-ribose being hydrogenated to N-D-ribityl-3,4-xylidine via xylidine riboside. Riboflavin can then be obtained from the N-D-ribityl-3,4-xylidine by reaction with diazotized aniline and barbituric acid.
The yields for the stages from D-glucose to N-D-ribityl-3,4-xylidine in this process are about 20-23%. This results in a total yield of riboflavin, starting from D-glucose, of 15-16% of theory. Besides the relatively poor total yield, the disadvantage of this process is also the epimerization stage from D-arabonic acid to D-ribonic acid, in which a number of resinous by-products are formed which make a complicated purification necessary. However, the amalgam reduction in particular presents enormous problems, since when large amounts of mercury are utilized, a considerable effort must be made to keep both the products and the waste substances free from mercury.
Processes which avoid the use of mercury have therefore already been proposed. Thus, D-ribonic acid or D-ribonolactone can be hydrogenated to N-D-ribityl-3,4-xylidine in the presence of 3,4-xylidine or 4-nitro-1,2-xylene in one stage. However, this process requires a very capital-intensive high pressure hydrogenation unit, since the hydrogenation proceeds under a pressure of about 250-300 bars. In spite of saving intermediate stages and in spite of the high expenditure, the yield of N-D-ribitylxylidine in this process, starting from D-glucose, is also only about at most 35%, which results in a riboflavin yield of less than 25%.
It has furthermore been proposed to prepare D-ribose directly from D-glucose by fermentation in a microbiological process with the aid of suitable microorganisms and then to obtain N-D-ribityl-xylidine and riboflavin from the D-ribose in the customary manner. Although this route has advantages, considerable difficulties which impair the profitability of the process are nevertheless to be expected when the biochemical process, which is complicated and above all susceptible to disturbances, is carried out on a large industrial scale. Moreover, the yield of N-D-ribityl-xylidine in this process, starting from D-glucose, is also only about 34%, which results in a riboflavin yield of about 24%.